The exemplary embodiment relates generally to the simulation of gloss inherent in the hardcopy of image data, be it pictorial or text. More particularly, it relates to the generation of a simulated differential gloss image for display or printing in which the impact on the background image is controlled.
Digital watermarks have been applied to images for a variety of purposes, including verification of authenticity of documents, providing information, and prevention of copying. Many of the techniques for generating such watermarks involve adding image data or modifying image data in an existing image prior to printing the image. In other techniques, selective application of a clear toner to a rendered image is used to create a watermark.
More recently, differential gloss (Glossmark™) technology has been used for the generation of digital watermarks. The technology allows a digital watermark to be introduced without modification of the original image data or the use of special marking materials or substrates. Rather, the technique creates a differential gloss image which can be discerned by a human reader holding the printed paper at an angle but which cannot be detected by a scanner or copier. These devices are restricted to reading the image at a fixed angle to the paper, which is generally selected so as to minimize the impact of specular reflection. The differential gloss image is created by selective use of two different halftone screens which create two different gloss levels in halftone outputs.
In halftone printing, a contone image (an image in which gray levels are substantially continuously variable) is converted to a halftone image by applying a screen. In a dot halftone system, the screen is essentially a grid in which the cells are either on or off, depending on the gray level. Each cell has a different gray level at which it is turned on. The screen determines how the dot cluster grows with increasing gray level. In conventional halftone dot screens, the cluster grows generally radially outward from a single cell or cells as gray level increases. In differential gloss techniques, two halftone screens having different anisotropic structure orientations, which may be provided by differences in dot growth patterns and/or screen orientations, are used. By toggling between the two halftone screens in generating the halftone image, differences in gloss are apparent when the rendered image is viewed at an angle.
Because the differential gloss watermark is generated using halftone screens, the watermark is not visible on a computer-screen or on a sub-sampled lower-resolution image. However, it is often advantageous for a user to be able to select the location of the differential gloss watermark with respect to the background image. Techniques have been developed for replacing the actual screen invisible differential gloss watermark with a screen visible image while simultaneously maintaining the integrity of the mark.
There are two stages in the creation of a differential gloss image where a simulation of the differential gloss watermark is of interest. First in the graphics interface during creation of the watermark, a preview image is generated for the computer screen in order to indicate how a gloss effect image, which provides the watermark, lines up spatially with the background image. In one method, a reduced-contrast version of the gloss effect image is added to the background image which would then indicate placement. A user can indicate the relative position of the gloss effect image via a user interface, which displays the gloss effect image superimposed upon the background image.
Second, when a reference to the created differential gloss image is placed into an applications document, a low-resolution placeholder image containing the reference is utilized. The placeholder image allows proofing for size, color, and placement both in the applications program and on simple prints from a conventional printer which is not enabled for creation of differential gloss images.
Existing methods have not been able to show the true colors of the background image as it would be seen straight-on, without the gloss effects, while at the same time simulating the gloss variation as it would appear with off-axis illumination. In particular, the existing techniques for simulating the placement of a differential gloss mark result in a marked alteration of the true colors of large portions of the background image. Further, there is no simulation or indication of the strength of the gloss effect. The differential gloss in a differential gloss print is less noticeable in some areas of a printed image, such as in high-light and deep-shadow regions. This is because the anisotropy in the halftone structures, which creates the differential gloss effect, is apparent at intermediate gray levels and diminishes rapidly to zero at either end of the grayscale. However, in the prior simulation technique, the added gloss effect image shows up equally strongly throughout the background image including in those high-light and deep-shadow regions where the differential gloss effect is very weak in the actual GlossMark prints.